Large-area low-dislocation-density column-III nitride substrates are needed for epitaxial growth of thin-film III-nitride materials for optoelectronic and high-frequency, high-power electronics. Current technology employs sapphire (Al2O3) or silicon carbide (SiC) as substrates, which are highly lattice-mismatched to GaN (+17% and −3.5%, respectively), and result in dislocation densities in the 109-1011 per cm range. Dislocation filtering mechanisms, such as epitaxial lateral overgrowth or cantilever epitaxy involve numerous difficult processing steps. These increase the cost of the manufacturing process and can also decrease yield. They typically reduce the total threading dislocation density by a factor of 102, which is not enough to reach the dislocation densities on the order of 105/cm2 or fewer that are desired for long-lived commercial devices.
For virtually all semiconductor materials, the largest-area lowest-dislocation-density substrates are grown by melt-based approaches. Hence, it would be desirable to apply such approaches to the III-nitrides. Indeed, for GaN, melt-based growth approaches are being pursued, either by dissolving nitrogen in a gallium melt (for example, see I. Grzegory, “High pressure growth of bulk GaN from solutions in gallium,” J. Phys. Condens. Matter 13 (2001) p 6875-6892), or dissolving gallium nitride in a liquid ammonia solvent, followed by precipitation of gallium nitride (for example, see D. R. Ketchum and J. W. Kolis, “Crystal growth of gallium nitride in supercritical ammonia,” J. Crystal Growth 22 (2001) p. 431-434). In both cases, however, enhancing the solubility of the gallium nitride precursors requires thousands of atmospheres of pressure. Even with such overpressures, the solubilities are low and the precipitation kinetics slow. Moreover, the pressure regimes (4,000 to 45,000 atmospheres) of these approaches will likely limit their scalability and manufacturability.
GaN single crystals have been prepared in a sealed stainless-steel tube container at 650-840° C. for 6-300 h using Ga, NaN3, and Na as starting materials. A thin GaN layer covered Na—Ga melt surface at the initial stage of reaction between Ga in the Na—Ga melt and N2 given by the thermal decomposition of NaN3 around 300° C. In the next stage, pyramidal and prismatic GaN single crystals grew under the layer. Prismatic and platelet crystals also grew from the melt which wetted the tube wall. The reaction rate was enhanced by increasing temperature and by increasing Na content in the melt. The maximum size of pyramidal crystals was about 0.7 mm. The platelet crystals were 1-2 mm in one direction and <0:05 mm thick (H. Yamane et al., “GaN single crystal growth from a Na—Ga melt,” J. Mater. Sci. 35 (200) p. 801-808).
The growth conditions for hexagonal GaN platelet crystals using a Li flux were reported by Y. T. Song et al., “Preparation and characterizations of bulk GaN crystals,” J. Crystal Growth 260 (2004) p. 327-330. The starting materials used for the growth of the GaN single crystals were Ga (99.999%) and Li3N (99.5%). These materials in proper proportion were put in a tungsten crucible and were heated to 800° C. in growth furnace which was charged with N2 gas of 1-2 atm at room temperature and then was slowly cooled at a rate of 2-3° C. per day. Hexagonal GaN platelet crystals can be obtained, and separated from residual substances by soaking in HCl solution. The experimental results confirmed that these crystals crystallized from Li—Ga—N liquid phase.
Liquid phase electroepitaxy (LPEE) is a method of crystal growth in which the layer growth is initiated and sustained by passing a direct electric current through the solution-substrate interface while the temperature of the overall system is maintained constant. This method may also be called electroliquid epitaxy (ELE) or current controlled liquid phase epitaxy (CCLPE). A review of LPEE is found in Golubev et al., “Liquid phase electroepitaxy of III-V semiconductors,” J. Crystal Growth 146 (1995) p. 277-282.
The successful melt-based bulk III-nitride growth approach will be one that is scalable, manufacturable, moderately inexpensive, and controllable. It should have a high growth rate and produce a low impurity content. It should produce crystals of superior crystalline quality (dislocation densities below 107 cm−2). It is also desirable that the technique be applicable to all of the III-nitrides of technological interest, that is, AlN, GaN and InN.